Mobility criteria in current wireless systems are typically based on downlink measurements of absolute received signal power or signal to noise/interference ratio. Mobility criteria include, for example, when to activate inter-Radio Access Technology (RAT)/frequency measurements and when to hand over the User Equipment (UE) to another base station or network node.
With regard to mobility, when in idle mode, the UE measures the neighbor cells and camps on the best cell. In connected mode, the UE measures and reports the best neighbor cells to the network and, based on these reports, the network decides when to perform a handover for the UE. Thus, in both idle mode and connected mode, the mobility of the UE is based on detection of neighbor cells by the UE. In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), the UE performs cell detection by searching for Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) transmissions. By detecting the PSS and SSS transmissions from a cell, the UE is able to find the correct cell timing (via PSS) and cell identity (via SSS) of the cell. In 3rd Generation (3G) wireless systems, cell detection is performed in a similar way.
In LTE, detection of a cell is, as is well known in the art, based on matched filtering using three PSS versions over at least 5 milliseconds (ms) of received samples. Correlation peaks in the filter output may reveal synchronization signals from one or more cells. This is referred to as symbol synchronization. Upon having established symbol synchronization and identified the cell-within-group Identity (ID) of the cell, the next step is SSS detection to acquire frame timing and physical layer cell identity. After decoding the SSS, the cell group ID and thereby the full physical layer cell ID is acquired. Moreover, frame timing and cyclic prefix configuration are determined. The pair of PSS and SSS is always transmitted from the same antenna port of the network node, but different pairs may be transmitted from different antenna ports, as defined in 3GPP Technical Specification (TS) 36.211 V12.3.0, Section 6.11.
One issue with cell detection is that interference can prevent detection of cells, particularly in situations where the interference is strong relative to the perceived strength of the PSS/SSS transmissions at the UE. This interference generally falls into two categories, namely, known interference and unknown interference. Known interference is interference that results from transmission of known signals from one or more interference sources. For instance, the known interference may be transmission of PSS/SSS by known cells (i.e., cells previously detected by the UE such as the PSS/SSS transmissions by a current or previous serving cell of the UE). Conversely, unknown interference is interference that results from transmission of unknown signals from one or more sources (e.g., transmission of unknown signals from other cells or UEs and/or transmissions from wireless nodes in other wireless systems such as transmissions from wireless nodes in a WiFi network).
For instance, when searching for a new cell in an LTE system that operates in a licensed frequency band, the received signal consists of signals from cells and, in Time Division Duplexing (TDD) modes, signals from other UEs. In LTE TDD, as well as in Time Division Synchronous Code Division Multiple Access (TDSCDMA), there are uplink and downlink timeslots which operate on the same frequency in a time-division manner. When a cell search is starting on a new band with LTE TDD, the UE does not know the timing of the uplink and downlink slots. The UE will thus receive interference from base stations transmitting in the downlink direction as well as UEs transmitting in the uplink direction. There are also discussions on having full duplex on the same carrier frequency; this will result in UEs transmitting continuously in the uplink that will cause interference in the downlink to another UE.
Further, the LTE system is planned to be deployed in unlicensed bands in the future. In this case, there may be other kinds of interference sources operating on the same frequency. These interference sources may be WiFi transmissions, Bluetooth, radar transmitters, etc.
Interference that degrades cell detection is particularly problematic in new modern deployment scenarios. For example, consider a scenario where the UE, due to strong interference, cannot detect new cells and, as a result, the possibility to change to another cell is limited. For dense deployments and small cell scenarios, it is necessary to detect weak neighbor cells for potential handover early in order to avoid long handover interruption or radio link failure. If a neighbor cell cannot be identified before the UE reaches the cell border, the likelihood of the UE dropping a data connection or a call increases, and the UE might not even be able to receive a handover command before losing connection to the serving cell.
Further, existing 3GPP requirements on mobility measurements and event triggering as captured in 3GPP TS 36.133 V12.4.0 have been derived with mobility at low speed in mind and for cells with a Signal to Interference Plus Noise Ratio (SINR) of −6 decibels (dB) or higher unless enhanced Inter-Cell Interference Coordination (eICIC) or further eICIC (feICIC)—both of which require tight coordination between cells—is used. A UE implementation that just barely fulfills those requirements will face big challenges in the new deployment scenarios described above.
As such, there is a need for systems and methods for improving cell search particularly in the presence of unknown interference without impacting UE complexity.